Oftentimes, during therapeutic procedures it is desirable to deliver a particular medical device or a therapeutic agent to a discrete location in the vasculature of a patient. Typically, such delivery is accomplished using a catheter which is introduced into a blood vessel of the patient and then advanced to the targeted location. Examples of such interventions include but are not limited to, angioplasty, stenting, determination of an embolization, local drug injections, including local chemotherapy, and gene therapy. Unfortunately, such procedures can be very difficult and time-consuming since the vasculature is highly branched. In addition, the extensive branching of the vasculature can lead to misdirection of the catheter.
In recent years, attempts have been made to use MRI to monitor the advancement of the catheter in the vasculature. The potential benefits of using magnetic resonance guidance for endovascular procedures have recently been advanced by a number of research groups .(Bakker C J, Hoogeveen R M, Hurtak W F, Van Vaals J J, Viergever M A, Mali W P. Radiology 1997; 202: 273-276; Omary R A, Frayne R, Unal O, Grist T M, Strother C M. J Vasc Interv Radiol 1999; 10: 1315-1321; Omary R A, Frayne R, Unal O et al. J Vasc Interv Radiol 2000; 11: 373-381; Spuentrup E, Ruebben A, Schaeffier T, Manning W J, Gunther R W, Buecker A. Circulation 2002; 105: 874-879; Wacker F K, Reither K, Branding G, Wendt M, Wolf K J. Mag. J Magn Reson Imaging 1999; 10: 841-844; Wildermuth S, Debatin J F, Leung D A et al. Radiology 1997; 202: 578-583; Strother C M, Unal O, Frayne R et al. Radiology 2000; 215: 516-519; Omary R A, Unal O, Koscielski D S et al. J Vase Interv Radiol 2000; 11: 1079-1085.) These include the lack of ionizing radiation, the avoidance of iodinated contrast material, and the ability to measure changes in end-organ function with MR. Accurate visualization of the target vessels is essential during MR-guided procedures. However, the inflow effect of blood does not always provide sufficient contrast for clear vessel delineation, and is dependent upon the orientation of the imaging slice relative to the vessel and direction of blood flow. Thus, MRI procedures using magnetic or paramagnetic substances that enhance contrast have been developed. The use of TI-shortening gadolinium preparations, such as Magnevist™ (Gd-DTPA, Schering A G, Berlin, Germany), provides a more flow-independent vessel visualization. However, Gd-DTPA has a very short intravascular retention time. Therefore, multiple injections are required to repeatedly obtain angiograms during an interventional procedure. This can cause marked enhancement of the background tissue, which reduces the vessel contrast and thus the overall image quality.
Because of the drawbacks of extracellular contrast agents, the use of intravascular paramagnetic contrast agents has received increased attention (Anzai Y, Prince M R, Chenevert T L et al. MR angiography with an ultrasmall superparamagnetic iron oxide blood pool agent. J Magn Reson Imaging 1997; 7: 209-214; Dong Q, Hurst D R, Weinmann H J, Chenevert T L, Londy F J, Prince M R. Magnetic resonance angiography with gadomer-17. An animal study original investigation. Invest Radiol 1998; 33: 699-708.) Intravascular contrast agents allow generation of high quality roadmaps of the vasculature, but, in contrast to extracellular contrast agents, also allow continuous visualization of the vessels for a certain amount of time, dependent mainly on the specific intravascular agent used (Bakker C J, Bos C, Weinmann H J. Passive tracking of catheters and guidewires by contrast-enhanced MR fluoroscopy. Magn Reson Med 2001; 45: 17-23; Wacker F K, Wendt M, Ebert W, Hillenbrandt C, Wolf K J, Lewin J S. Use of a blood-pool contrast agent for MR-guided vascular procedures: feasibility of ultrasmall superparamagnetic iron oxide particles. Acad Radiol 2002; 9: 1251-1254.) However, these methods have not always permitted accurate visualization of the target vessels, particularly accurate visualization of the location of the catheter and the vessels immediately downstream of or distal to the catheter. Accordingly, it is desirable to have additional MRI-based methods and systems for monitoring the location of the tip of an intravascular catheter and the progress and success of an intervention.